Isochrons in presolar graphite grains from orgueil

Abstract

Primitive meteorites contain tiny dust grains that condensed in stellar outflows and explosions. These stardust grains can be extracted from their host meteorites and studied in detail in the laboratory. We investigated depth profiles of the Al-Mg, Ca-K, and Ti-Ca isotopic systems obtained during NanoSIMS isotopic analysis of presolar graphite grains from the CI carbonaceous meteorite Orgueil. Large 26Al/27Al, 41Ca/ 40Ca, and 44Ti/48Ti ratios, inferred from 26Mg, 41K, and 44Ca excesses from the decay of the short-lived radioisotopes 26Al, 41Ca, and 44Ti, indicate a supernova (SN) origin. From the depth distribution of the radiogenic isotopes and the stable isotopes of their parent elements we constructed isochron-type correlation plots. The plots indicate quantitative retention of radiogenic 26Mg, 41K, and 44Ca in most grains. Deviations from straight lines in the Al-Mg and Ca-K plots can be explained by contamination with 27Al and isotopically normal Ca, respectively. For the Ti-Ca system in some grains, the lack of parent-daughter correlation indicates either redistribution of radiogenic 44Ca or heterogeneity in the initial 44Ti/48Ti ratio. We also obtained Si isotopic depth profiles in three graphite grains with large 29Si and 30Si excesses, for which a SN origin has been proposed. In two grains no Si-rich subgrains are observed; in the third grain with an apparent Si-rich subgrain the anomalous Si isotopic ratios in the subgrain are the same as in the rest of the graphite host. Our studies show that by measuring depth profiles, information on presolar grains can be obtained that cannot be obtained by whole-grain analysis. © 2013. The American Astronomical Society. All rights reserved.